For many years now, one common form of electronics test equipment used by electronics designers has been signal sources. One type of signal source of particular importance is the pulse generator. Typically, pulse generators are designed to produce an electronic pulse signal consisting of a series of one or more pulses, such as a voltage pulse VP shown in FIG. 1. Normally, various parameters describing the pulse signal, such as its amplitude Vpp, positive edge (rise) time TpET associated with a rising edge 1, negative edge (fall) time TnET associated with a falling edge 2, frequency (not denoted), and duty cycle (also not denoted), are selectable by the pulse generator user. Additionally, pulse generators typically operate in several different voltage amplitude ranges, such as zero to 0.1 volts, zero to one volt, zero to ten volts, and so on, with the smaller ranges generally providing greater voltage resolution than larger ranges.
Specifically with respect to the programmability of the pulse rise and fall times, user selection of the positive and negative edge times TpET, TnET of the pulse VP allows variable edge control of the pulse generator. Typically, the positive edge time TpET of the voltage pulse VP is defined as the time required for the voltage to rise from ten percent to ninety percent of its amplitude Vpp (i.e., 0.1 Vpp to 0.9 Vpp) as shown in FIG. 1. Similarly, the negative edge time TnET of the pulse VP is commonly defined as the time require for the voltage to fall from ninety percent to ten percent of its amplitude Vpp.
Users of pulse generators currently desire at least two characteristics of user-selectable variable edge control. First, a pulse generator should have the capability to maintain balanced, or substantially equal, positive and negative edge times TpET, TnET as specified by the user so that less user manipulation of the pulse generator is required. Furthermore, the positive and negative edge times TpET, TnET selected by the user should be held substantially constant while the amplitude Vpp is altered through a large dynamic range so that less user manipulation of the generator is required when changing the amplitude Vpp of the pulse VP.
Currently, some pulse generators employ variable edge control that exhibits these attributes. Those generators that provide these advantages normally require a calibration process whereby two different edge times within each voltage range are used. The edge times are normally set by way of a digital value resident in a digital-to-analog converter (DAC) for each of the negative and positive edge times. These four DAC values (a negative and positive edge time value for each of the two edge times used) are then employed to calculate a gain and offset value for the particular voltage range. The gain and offset values are then used to determine DAC values for other positive and negative edge times within that voltage range to maintain balanced variable edge control.
Given some electronic circuit differences between each voltage range, such a method of calibration normally must be performed separately for each voltage range of the pulse generator. Additionally, since the calibration is performed independent of the actual voltage amplitude of the pulse being used, such a calibration is effective only if pulse amplitude and edge times are independent. In other words, if altering the pulse amplitude also alters the positive or negative edge times without a change in the DAC value associated with the edge times, pulse amplitude and edge time are interrelated, and the aforementioned calibration procedure will not operate effectively on such a pulse generator.
From the foregoing, then, a need exists for an improved variable edge control method that applies to a wider range of pulse generation circuits.